1. Field of the Invention
This invention relates to a sheet processing system and, more particularly, to a system for producing a continuous polymer sheet and winding it on a take-up roll, while providing for smooth and trouble-free take-up roll changes. Specifically, the invention provides a novel construction and arrangement for accumulating the continuously produced polymer sheet while the take-up roll is being changed on the fly.
2. Discussion of the Prior Art
Continuous sheet processing systems are known that provide for take-up roll changes. It is also known to be necessary to provide some form of accumulator between continuous and intermittent in-line processes.
U.S. Pat. No. 3,904,338 to Straumanis discloses a system and method of controlling a machine which continuously produces sheet that is supplied to an intermittently activated in-line processor. The system involves regulation of the sheet extrusion speed along with a vertically oriented and pivotally mounted dancer roller which acts as an accumulator.
Other references disclose various machines for accumulating and temporarily storing a loop of continuous strip material, using a roller loop carriage mounted on a track or cage assembly.
With some continuous sheet processing systems, a plastic sheet is extruded and cooled via a multiple roll stack (sometimes erroneously called a calendar), or a sheet take-off. It is subsequently run over a more or less horizontal stretch of idler rolls for further cooling, thickness measurement, surface treatment, etc. It is then usually edge trimmed to desired width, passed through a set of pull rolls and then wound into rolls on a winder.
These take-up rolls must be changed when they grow to a desired diameter. This creates a problem, especially at increasingly higher speeds because it is difficult to cut the web and transfer it to a new winding shaft in the short time available. This is also dangerous because operators have, at times, been injured in the transfer.
Automatic roll transfer systems exist but are limited to relatively thin plastic sheet because of their limited ability to cut thick plastic sheets. Besides, they are quite expensive. The thickest plastic sheet automatic cut-off system known to be available can cut up to about 0.030" (0.75 mm) thick sheet. But today it is often needed to wind sheet up to 0.80" or 0.100" (2-2.5 mm), or more.
Another frequently used device is a sheet accumulator between the sheet take-off and the winder. This uses a series of (usually vertically arranged) pairs of moveable rolls to accumulate the extruded sheet while the winder is slowed down or even stopped. These accumulators have several disadvantages:
1. They take up excessive valuable space.
2. They are expensive.
3. They require edge trimming after passing through the accumulator because the sheet is never perfectly straight and its edges tend to move laterally as the accumulator stores more or less material.
4. One further great disadvantage arises in building and running compact sheet lines wherein the winder is directly integrated into the downstream end of the sheet take-off. There is no room upstream on a sheet take-off system for the heavy and bulky conventional accumulator.